This invention relates generally to sealing of openings of vessels and containers and more particularly to a process for affixing cover materials in sealed state onto the opening rims of glass containers.
It is a general practice to place a powder material such as coffee or a dry dairy product in a glass container such as a jar and to affix a cover material comprising a piece of metal foil and a resin layer formed on a surface thereof onto the rim of the container opening thereby to seal the powder material in the container.
For affixing the cover material in sealed state onto the opening rim of the glass container, the induction sealing process has heretofore been widely used.
This induction sealing process comprises causing each of a plurality of glass containers on the opening rim surface of which a cover material (hereinafter referred to as "cover") has been placed to pass through a region where it is exposed to high-frequency electromagnetic waves thereby to cause its resin layer to be heated by its self-induced heat, whereby the resin layer is melted and caused to adhere to the opening rim surface. A capping machine carrying out such an induction sealing process is capable of high-speed operation of sealing approximately 500 containers per minute and is therefore being utilized for sealing products such as powdery or solid food products other than food products in liquid form.
However, it has heretofore been impossible to carry out satisfactory sealing of liquid products by this induction sealing process. The reason for this is that, since a liquid product is sterilized by heating and charged into glass containers at high temperature, and the vapor pressure of the food product or the like in the glass container gives an interior pressure of the container at the time of sealing and exerts a force tending to cause the heat-sealed part of the cover to separate from the container rim surface immediately after sealing, thereby giving rise to instability of the adhesive or bonding strength of the sealed parts.
Another process for affixing a cover is the heat-sealing process wherein a cover is heated by means of a heating plate and thereby caused to be affixed by heat adhesion to the opening rim surface of a glass container.
However, in general, the heat capacity of a glass container is great, and the cover cannot be caused to adhere positively to the opening rim surface merely by applying heat to the cover from the heating plate including the heat transmitted to the glass container. Particularly in the case of most glass containers, each container is formed into an integral structure by joining together two halves thereof, and in many cases projections of the order of 0.2 to 0.3 mm exist on the opening rim surface in the vicinity of the joint between the container halves. The presence of these projections impairs the adhesion given by heating and pressure application.
In the case of a cover, such as an intermediate or inner cover of a container, which comprises a thin metal foil of a thickness of the order of 100 microns or less and a sealant layer also of a thickness of the order of 100 microns or less laminated onto the foil, projections of this character on the opening rim surface of the container cannot be absorbed by the cover, whereby portions where the applied force is locally not transmitted are created. This defective state can be corrected by providing a cushioning layer of a material such as rubber of a thickness of the order of 2 to 5 mm on the contacting surface of the heating plate, but a presence of such a cushioning layer, itself, obstructs the transmission of heat from the heating plate to the cover. Therefore, the heat-sealing process is still not a satisfactory process on the points of adhesive strength and work efficiency and has not yet been reduced to effective practice.